In a semiconductor avalanche photodetector, the noise depends upon the ratio of the ionization coefficients of the two types of charge carriers present in semiconductor devices and on the mechanism which initiates carrier multiplication. The ionization coefficient is the probability of the occurrance of an ionizing collision per unit length. A large difference between the ionization coefficients of the electrons and holes results in desirable noise characteristics for an avalanche photodetector, provided that the avalanche is initiated by the carrier type having the larger ionization coefficient. In the ideal avalanche detector, only one type of charge carrier would undergo ionizing collisions. [See McIntyre, R. J., IEEE Transactions on Electron Devices, Vol. 13, p. 194 (1966).]
Accordingly, efforts have been directed to developing an avalanche photodetector in which one type of charge carrier has a larger ionization coefficient than the other and in which the avalanche is initiated by the type of charge carrier having the larger ionization coefficient.
In contrast with silicon, many group III-V semiconductor materials exhibit substantially equal ionization coefficients for electrons and holes. It is therefore advantageous to tailor the structure of a photodetector comprising group III-V semiconductors so that the two species of charge carriers do not ionize at substantially the same rate and the avalanche discharge is initiated by the type of charge carrier having the larger ionization coefficient.